The invention disclosed herein relates generally to a close loop position control system, for example, for positioning a printing device in an electronic postage meter. More particularly, the invention relates to an actual incremental position encoder and incremental encoding method for such a system.
Known digital position control systems for controlling a d.c. motor may include a single stage, two channel incremental encoder which provides in response to motor shaft rotation two electrical signals 90 electrical degrees out of phase, i.e., in quadrature. The two signals together provide N quadrature states per motor shaft rotation. Shaft position is determined by counting the quadrature states and rotation direction is determined from the phase of the two signals, i.e., which signal leads (or lags) the other. For example, the following U.S. patents, all assigned to the assignee of this application, disclose digital controllers employing an incremental quadrature encoder of the type described above: U.S. Pat. Nos. 4,630,210; 4,631,681; 4,635,205; 4,636,959; 4,646,635; 4,665,353; 4,638,732; and 4,643,089; all issued to Salazar et al, and U.S. Pat. No. 4,774,446; issued to Kirshner et al. The disclosures of all of those patents are incorporated herein by reference.
These patents each disclose an electronic postage meter including a computer controlled d.c. motor use to control print wheels of the electronic postage meter. The control loop for the d.c. motor is implemented by the computer and software, except for the amplifier driving the d.c. motor, an incremental quadrature encoder of the type described above, and external counting circuitry for decoding the signals from the quadrature encoder. External decoding circuits are currently available as monolithic integrated circuits, for example, DHC 2000 from Texas instruments Incorporated. If desired, the quadrature signals may be decoded by the computer. Whether an external circuit or the computer decodes the quadrature output signals of the encoder depends upon whether the computer's internal counting circuit is available or is being used for other purposes.
The computer and the software calculate and apply, to the amplifier for the d.c. motor, pulse width modulated (PWM) drive signals utilizing a digital compensator derived from an analysis of the d.c. motor, the motor load, and other control loop components. The d.c. motor is driven according to a predetermined motion profile. The encoder and external counting circuitry for decoding the quadrature signals provide digital signals to the computer and the computer provides digital PWM drive signals so that analog-to-digital and digital-to-analog converters are not required in the d.c. motor control loop.
In order to attain high positional accuracy, digital controllers of the type described above sample at 1 ms or less intervals, which requires the controllers to have a motor control bandwidth of 1 KHz or greater. Such bandwidths are relatively high and increase the cost of the controller.
The control loop in the digital controllers in the above patents utilizes a single stage, two channel, quadrature incremental shaft encoder which includes a transparent disk having a plurality of opaque lines formed at equidistantly angularly-spaced intervals along at least one of the disc's opposed major surfaces, and an optical sensing device for serially detecting the presence of the respective opaque lines as they successively pass reference positions.
In response to detecting the presence of the opaque lines, the encoder provides two output signals on two separate lines or channels in quadrature.
The incremental quadrature shaft encoders described above are of high resolution and are used in high bandwidth systems. Both the shaft encoders and the digital controller employing them tend to be expensive and, because of the high bandwidth requirements, the digital controllers are frequently dedicated to motor control.
Another single stage, two channel incremental quadrature encoder is described in application Ser. No. 423,822 , and in application Ser. No. 423,813, both referenced above.
The incremental quadrature encoder disclosed in the above applications is of low resolution and is relatively inexpensive. However, it is used in a lower cost, lower accuracy, digital controller for applications where high controller bandwidth is not a requirement. For example, the digital controller described in the two patent applications referenced immediately above operates with a 2.5 ms sampling period (400 Hz bandwidth). Those patent applications describe a digital controller which effectively simultaneously controls five d.c. motors so that five print wheels coupled to respective d.c. motors may effectively be set in parallel. The controller described in those applications accomplishes control of the five d.c. motors utilizing a low cost, low-power microcomputer including a single low-cost, low-power microprocessor (8-bit). Yet, because of the low bandwidth requirements, the microcomputer is able to control all other postage meter functions including postage accounting, printing, fault monitoring, etc.
While expensive, high resolution incremental quadrature encoders are available for use in high bandwidth, high accuracy control systems, and inexpensive, low resolution incremental encoders are available for lower bandwidth, lower accuracy systems, there is a need for an inexpensive, high resolution incremental shaft encoder for use in low (or high) bandwidth, high accuracy control systems.